This invention is related to communication systems, and more particularly to wireless communication systems.
A transmitter uses an antenna to transmit a signal to a receiver. For example, a transmitter in a so-called base station of a wireless communication system uses an antenna to transmit a signal to a mobile terminal. The antenna emits energy to generate an electromagnetic field (EM field) that carries the signal to the mobile terminal. A phased-array antenna, in particular, generates an EM field that is typically more focused than the EM field generated by a non-phased-array antenna. By a “more focused EM field” is meant an EM field where 1) the largest amount of energy is directed in a particular azimuth direction—for example, the azimuth direction of the mobile terminal to which the signal is being transmitted—, and 2) as the angle from the azimuth direction increases, the strength of this EM field drops off more sharply than the strength of the EM field that is not focused, such as an EM field generated by a non-phased array antenna. The azimuth direction is the angle of the mobile terminal from the broadside, the vertical plane perpendicular to the front face, of the phased-array antenna. Although it is actually the electromagnetic energy that is directed in a particular azimuth direction, for ease of reference it is the EM field that is referred to herein as being directed in the azimuth direction in which the largest amount of energy is directed.
Because the EM field generated by the phased-array antenna is more focused (also sometimes referred to in the art as being “narrower”) than the EM field that would be generated by a non-phased array antenna, then, the signal carried by the EM field directed by the phased-array antenna to a particular mobile terminal interferes less with signals to other mobile terminals in the same, or other, so-called sectors of the cells of the wireless communication system than the signal carried by the EM field that would be generated by the non-phased-array antenna. This allows an increase in the number of mobile terminals in the wireless communication system, and therefore an increase in the capacity of the wireless communication system. (The capacity of the wireless communication system is the number of calls that can be carried simultaneously by the wireless communication system.)
The phased-array antenna should generate the EM field so that an acceptable receive voltage—the voltage received by the mobile terminal—is induced at the location of the mobile terminal. The acceptable receive voltage is that voltage necessary for the mobile terminal to receive the signal with an acceptable level of signal performance. Typically, power control information or quality-of-signal information, such as error information bits, received from the mobile terminal is used at the base station to adjust the EM field so that an acceptable receive voltage is induced at the location of the mobile terminal.
In a typical wireless communication system, signals transmitted for a number of mobile terminals are all transmitted at the same frequency. (For example, in a so-called TDMA wireless communication system, signals to three mobile terminals are transmitted on the same frequency.) Using a phased array antenna to transmit a signal on the same frequency to several mobile terminals could result in destructive interference of the EM fields directed to the several mobile terminals. Destructive interference occurs when an EM field directed to a first mobile terminal induces a secondary voltage at the location of a second mobile terminal, where this secondary voltage is out of phase with the primary voltage induced by the EM field directed to the second mobile terminal. In this case, the secondary voltage will reduce the magnitude of the primary voltage. This reduction in magnitude may be large enough so that the second mobile terminal may not receive an acceptable receive voltage, and therefore may not receive the signal with an acceptable level of signal quality.
It has been proposed to minimize the problems due to destructive interference by using focused EM fields having alternating orthogonal polarization, each EM field being directed in a fixed direction. The voltage induced at the location of a mobile terminal is induced by both the EM field whose direction is closest to the direction of the mobile terminal and by the all the other EM fields. However, since adjacent EM fields have orthogonal polarization, the possibility of destructive interference is reduced. This reduces the possibility that one of the mobile terminals will not acceptably receive the signal—receive the signal with an acceptable level of signal performance.